Surface polysaccharides of pathogenic bacteria, including capsular polysaccharides (CP) or the O-specific polysaccharide (OSP) of lipopolysaccharides (LPS), function both as essential virulence factors and protective antigens. Covalent binding of these saccharides to medically useful proteins to form conjugates increases their immunogenicity and confers upon them T cell dependence, making them suitable vaccines for infants and children. The O-SP of Shigella sonnei bound to recombinant non-toxic P. aeruginosa exoprotein A (rEPA) had an efficacy of over 70 percent in young adults exposed to 6 to 14 percent attack rates. This conjugate and that of S. flexneri 2a bound to the succinylated exoprotein A (rEPA-succ) were safe and induced IgG antibodies to the homologous LPS in 1- to 4-year-olds. A randomized, blinded, Phase 3 study of the conjugates in 1- to 4-year-olds, with each conjugate serving as a control for the other, showed the vaccines to be safe. Immunogenicity and efficacy of the S. sonnei conjugate were age-related with no efficacy in 1- to 2-year-olds but with about 70 percent in 3- to 4-year-olds. As before, fold increases in antibody levels were similar to those for adults, but the actual achieved levels were lower. There were too few cases of S. flexneri 2a infection for statistical analysis. Protection from non-vaccine types of S. flexneri, in S. flexneri 2a conjugate recipients, was noticed. These types included type 6, the most common S. flexneri isolate during the study. The overall efficacy of S. flexneri 2a vaccine against all S. flexneri non-type 2a was 44.9%, and against type 6 alone was 51.7%;both these values were not statistically significant. More immunogenic vaccine candidates were prepared using the model of synthetic S. dysenteriae type 1 saccharides-protein conjugates. Low molecular mass O-SP-core (O-SPC) fragments containing an average of 3-4 repeat units (RU) of S. sonnei LPS were isolated and bound to carrier proteins. Levels of IgG anti-S. sonnei LPS induced by these conjugates in young outbred mice were significantly higher than those induced by the full-length O-SP conjugates. A clinical lot of this vaccine candidate was prepared. The applicability of this approach to S. flexneri types 2a and 6 and S. dysenteriae type 1 is being investigated. The structures of the O-SPC isolated from these bacteria, containing core plus 1-4 O-SP repeat units (RU), were analyzed by NMR and mass spectroscopy. The first RU attached to the core of S.flexneri 2a and 6 LPSs were different from the following RUs in their O-acetylation and/or glucosylation. Conjugates of core plus more than 1 RU were needed to induce LPS antibodies in mice. These antibody levels were comparable to those induced by the full length O-SP conjugates. In S. dysenteriae type 1, the first RU was identical to the following RUs, with the exception that the GlcNAc was bound to the core in the beta-configuration, while in all other RUs it was in the alpha-configuration. In spite of this difference, conjugates of S. dysenteriae type 1 core with 1, 2 or 3 RUs induced LPS antibodies in mice with levels similar to those induced by the synthetic S. dysenteriae type 1 saccharide conjugates and statistically higher than those of the full size O-SP conjugates. O-SPC conjugates are easy to prepare, characterize, and standardize, and their clinical evaluation is planned.